Polychlorobiphenyls (hereinafter referred to as PCBs) were used in a wide variety of applications such as insulating oil, machine oil, plasticizers and paints, because of excellent insulating properties, good solubility in oils and low vaporizability thereof. In 1966, PCBs, which are harmful to human bodies and chemically extremely stable, were found to cause environmental pollution and to be stored in a high concentration within living beings through food chains. Thus, early in 1970s, the manufacture of PCBs was prohibited. As a result, a total of about 10.sup.6 tons of PCBs have been stored in the world. There is a great demand for the establishment of technology for decomposing PCBs.
Known methods hitherto proposed for the decomposition of PCBs include (a) combustion at a temperature of at least 1,100.degree. C., (b) catalytic hydrogenation, (c) photochemical decomposition with UV rays, and (d) biological decomposition using microorganisms. These methods are not fully satisfactory with respect to energy consumption, treatment time, equipment cost or formation by-products.
JP-B-2590421 discloses a method of decomposing PCBs, wherein PCBs are contacted with an aqueous solution containing an oxidizing agent at a temperature of 623 K-823 K and a pressure of 20-60 MPa. The presence of the oxidizing agent is described as being essential in order to effectively decompose PCBs. This method is promising because PCBs can be effectively decomposed within a relatively short period of time but has a problem because the oxidizing agent cause corrosion of the treatment apparatus and decomposition of other useful materials such as oil in which PCBs are contained.